Allow filtering objects in `git-cat-file(1)`

It is not possible to filter objects in git-cat-file(1) right now, so if we want to enumerate all objects of a certain type we have to enumerate them all and then filter them manuall. This is inefficient, so we should investigate whether we can adapt git-cat-file(1) to use filters. Ideally, these filters would be similar to the filters we already have in git-rev-list(1). In practice though, only a subset of those filters would apply in the first place, so this may be hard to realize.